v0.2.0-beta

New features:

• The values of mutable cells can now be set directly with the
  assignment operator:

  auto a = live_cells::variable(0);
  
  std::cout << a.value() << std::endl; // 0
  
  a = 10;
  std::cout << a.value() << std::end; // 10

  The values can also be changed using the ++, --, +=, -=,
  etc. operators.

• Reactive cell pipelines with the | operator:

  The namespace live_cells::ops provides functions that can be
  applied on cells using the | operator:

  The following:

  auto cell = live_cells::peek(
      live_cells::on_error(
          live_cells::select(cond, a, b),
          c
      )
  )

  Can now be written as the following:

  auto cell = cond
      | live_cells::ops::select(a, b)
      | live_cells::ops::on_error(c)
      | live_cells::ops::peek;

Bug fixes:

• Fixed bug: MutableCell concept not being satisfied by any mutable
  cell.